Single frequency, two feed dish antenna having switchable beamwidth

ABSTRACT

A switchable beamwidth antenna includes a concave parabolic main reflecting dish which has a central circular region and a surrounding coaxial annular region. A feed means selectively excites only the central region of the main dish via a truncated subreflector for wide beamwidth or substantially the entire main dish for narrow beamwidth. In the embodiment shown, the feed means comprises a truncated concave ellipsoid subreflector and separate feed terminations located at two foci of the ellipsoid. One feed termination directly views all of the main dish while the other feed termination, exciting the main dish via the subreflector, excites only the central region because of the subreflector truncation.

United States Patent [191 Schmidt Dec. 16, 1975 1 SINGLE FREQUENCY, TWOFEED DISH ANTENNA HAVING SWITCHABLE BEAMWIDTH [75] Inventor: Richard F.Schmidt, Seabrook, Md.

22 Filed: Oct. 4, 1974 211 Appl. No.: 512,825

[52] U.S. Cl. 343/779; 343/837; 343/876 [51] Int. Cl. HOlQ 19/12 [58]Field of Search 343/779, 781, 837, 840

[56] References Cited UNITED STATES PATENTS 3,394,378 7/l968 Williams etal. 343/779 3,438,041 4/1969 Holtum, Jr 343/779 Primary Examiner-Paul L.Gensler Attorney, Agent, or FirmR0bert F. Kempf; Ronald F.

Sandler; John R. Manning 57 ABSTRACT A switchable beamwidth antennaincludes a concave parabolic main reflecting dish which has a centralcircular region and a surrounding coaxial annular region. A feed meansselectively excites only the central region of the main dish via atruncated subreflector for wide beamwidth or substantially the entiremain dish for narrow beamwidth. In the embodiment shown, the feed meanscomprises a truncated concave ellipsoid subreflector and separate feedterminations located at two foci of the ellipsoid. One feed terminationdirectly views all of the main dish while the other feed termination,exciting the main dish via the subreflector, excites only the centralregion because of the subreflector truncation.

2 Claims, 4 Drawing Figures MICROWAVE SOURCE OR RECEIVER 39- BEAMWIDTHCOMMAND U.S. Patent Dec. 16, 1975 Sheetlof2 3,927,408

FIG. 1

FIG. 2

MICROWAVE SOURCE OR RECEIVER 39 BEAMWIDTH COMMAND MICROWAVE SOURCE ORRECEIVER 39- BEAMWIDTH COMMAND US. Patent Dec. 16, 1975 Sheet 2 of23,927,408

FIG. 3

BEAMWIDTH 50 Y (degrees) ELLIPSOID DIAMETER (feei) SINGLE FREQUENCY, TWOFEED DISH ANTENNA HAVING SWITCHABLE BEAMWIDTH FIELD OF THE INVENTION Theinvention relates generally to switchable beamwidth or zoomable antennasand more particularly to a switchable beamwidth antenna employing acommon main reflecting dish operable in at least two different beamwidthmodes.

ORIGIN OF THE INVENTION The invention described herein was made by anemployee of the United States Government and may be manufactured andused by or for the Government for governmental purposes without thepayment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION The need frequently arises to augment narrowbeamwidth (narrow field of view) transmitting and/or receiving antennaswith a wide beamwidth acquisition mode. Since it is well known thatantennas are reciprocal, having the same characteristics fortransmitting as for receiving, the meaning of acquisition shall bedetailed with respect to a receiving antenna with the understanding thatacquisition for narrow beamwidth transmitting antennas and for narrowbeamwidth transmitting/receiving antennas such as a radar, issubstantially similar.

In the case of a narrow beamwidth (also referred to as high antennagain) receiving antenna there is great difficulty in pointing theantennas narrow field of view in the direction of a transmitting stationwhich must be done in order for the antenna to receive. If, at thesacrifree of antenna gain or efficiency, the antenna is initiallyswitched to a wide field of view (wide beamwidth), the antenna is moreeasily pointed to subtend the transmitting station in the wide field ofview. Then, an indication of pointing error may be derived bysimultaneous lobing techniques, for example, to more precisely point theantenna. Once the antenna is pointed so that the transmitting stationwould be in its narrow field of view, acquisition is said to haveoccurred and the antenna may be switched to its narrow beamwidth mode totake advantage of greater antenna gain or efficiency. This narrowbeamwidth may then be maintained, subtending the transmitting station(or tracking) by simultaneous lobing techniques. Similar acquisition maybe done to point a radar antenna at a target or a transmitting antennaat a receiving station.

The acquisition problem is particularly acute for narrow beamwidthantennas having large main reflector dishes of the type considered bythe National Aeronautics and Space Administration for Tracking and DataRelay Satellites to relay to earth the data collected from orbitingearth observation satellites. These antennas, operating at Gigahertz,would have a main dish on the order of 12.5 feet in diameter with aconsequent narrow beamwidth of only 0.3 degrees. Initial pointing of thenarrow beamwidth antenna of the Data Relay Satellite toward an EarthObservation Satellite would be quite difficult to achieve because ofsignificant rela tive motion between these satellites. Thus, a means forincreasing the beamwidth of the antenna to effect acquisition isrequired.

Numerous techniques were considered and found to be unsatisfactory.

In one technique either a feed or a subreflector is axially shifted inposition to defocus the antenna. This technique is not acceptablebecause, though the beamwidth is generally widened, the antenna patternamplitude and phase characteristics are distorted. In another technique,a polarization sensitive grating is placed in front of the main dish toserve as a smaller main dish for a wide beamwidth mode. This grating,though smaller than the dish, is sufficiently forward to intercept allradiation coming from a feed. The grating passes, for example,vertically polarized radiation to the main dish, producing a narrowbeamwidth but reflects horizontal polarization producing widerbeamwidth. Thus, beamwidth can be switched by switching feedpolarization. This technique suffers from restrictions on feedpolarization; in particular, it does not permit the use of circularpolarization which has both horizontal and vertical polarizationcomponents.

Another technique for increasing the beamwidth of the antenna is tochange the frequency of operation. Since beamwidth is inverselyproportional to the area of the main dish measured in wavelengths, thebeamwidth may be decreased by decreasing frequency (increasingwavelength). This is an undesirable antenna system complication forsatellite users which may also necessitate additional antenna feeds andconsequent increased blockage of the satellite main dish, which causes adecreased antenna gain or efficiency of the antenna. Moreover, it isdesirable to interface with existing user single frequency equipment.

Still another technique for increasing beamwidth is to provide two feedsat one feed point with one feed exciting the entire main reflector fornarrow beamwidth and a second feed exciting a smaller region of the mainreflector for wide beamwidth. There are many difficulties with thisapproach the chief one being that if the second feed is to besufficiently directive to excite only a portion of the main reflector,it would have to be geometrically large; such a large feed wouldsignificantly increase blockage of the main dish reflector decreasingantenna gain or efficiency. Furthermore, there are obvious difficultiesin placing two feeds at the same point; the second feed must bedisplaced from the antenna axis if the first feed is located on theaxis. It is desirable to have the capability of positioning the feed orfeeds in both wide and narrow beamwidth modes on the axis of theantenna.

OBJECTS OF THE INVENTION It is an object of the present invention toprovide a new and improved switchable beamwidth antenna in whichbeamwidth switching is independent of feed polarization, feeddirectivity, or feed frequency.

It is a further object of the present invention to provide a new andimproved switchable beamwidth antenna employing a main reflecting dishand feed therefor with a narrow beamwidth mode and a wide beamwidthacquisition mode wherein the feed or feeds in both modes can excite themain reflecting dish from the antenna axis.

It is yet a further object of the present invention to provide a new andimproved switchable beamwidth antenna allowing flexibility in feeddesign, for example, permitting the use of electronically ormechanically scanned feed arrays for scanning the viewing direction ofthe antenna.

SUMMARY OF THE INVENTION The present invention includes a switchablebeamwidth antenna having a main reflecting concave parabolic dish andfeed means effectively at a focal point on the dish boresight axis.Since the beamwidth of an antenna of this type is inversely proportionalto the main dish area, the beamwidth can be increased by operativelyusing only a portion of the main dish. For wide beamwidth operation, thefeed means excites only a central circular region of the main dish via atruncated subreflector while for the narrow beamwidth mode the entiredish is excited by the feed.

The invention has one main embodiment.

In this embodiment, electrical beam switching is attained by providing aGregorian configuration wherein a truncated concave ellipsoidsubreflector has a major axis located on the boresight axis of theparabolic dish. The ellipsoid subreflector has two foci in front of thesubreflector, whereby the subreflector focus nearest the subreflector iscoincident with the focus of the main dish. A separate feed is providedat each subreflector focus and may be selectively activated to providebeam switching. Another method of beam switching or zooming using asingle feed is disclosed in US. Pat. No. 3,866,233. A first feed,located at the main dish focal point, faces the main dish and excitesits entire surface. The second feed, located at the focus of theellipsoid furthest from the subreflector, faces the subreflector andexcites the main dish via the subreflector. The ellipsoid has thecharacteristic that a real image of the second feed is formed at itsnearer focus whereby, to the main dish, the second feed also appears tobe at the focus of the main dish. Because of the effective truncation orreduction in size of the ellipsoid subreflector, the second feed excitesonly a central region of the main dish via the subreflector.

The above and still further objects, features, and ad vantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of several specific embodiments thereof,especially where taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic drawing incross-section of the embodiment of the switchable beamwidth antennahaving a main dish and a truncated subreflector, wherein the antenna isshown in the narrow beamwidth mode;

FIG. 2 is a schematic illustration of the embodiment of FIG. 1 in thewide beamwidth mode;

FIG. 3 is a design graph indicating the antenna beamwidth for theembodiment of the switchable beamwidth antenna in the wide beamwidthmode of FIG. 2, versus the subreflector diameter (or degree oftruncation); and

FIG. 4 is a schematic drawing in front view of a multifrequency feed.

DETAILED DESCRIPTION OF THE INVENTION FIGS. 1 :and 2 are cross-sectionalillustrations of an embodiment 49 of the switchable beamwidth antenna ofthe invention wherein two feeds 51 and 53 and a truncated or reducedsize concave ellipsoid subreflector 55, located on the antenna boresightaxis 20, cooperate with the main reflecting concave paraboloidal dish ina Gregorian configuration. Feed 53 is located at the main dish focalpoint 27 facing the main dish. In

4 FIG. 1, source 21 supplies the feed 53 with radio frequency ormicrowave. radiation via conduit 23 and microwave switch 57 in responseto'a narrow beamwidth command issued to switch 57. Feed 53 directlyilluminates the main dish 15 with a cone of radiation having theincluded angle 6 bounded byrays 59a and 61a, which optimally justilluminates the entire main dish. Upon reflection of this cone by maindish 15, a collimated output beam, bounded by rays 59b and 61b, isproduced having an initial large radius. equal to the dish radius, and acorresponding narrow beamwidth given by diffraction theory.

In FIG. 2, feed 51, facing the ellipsoidal subreflector 55, illuminatesthe main dish via the subreflector to produce a wide beamwidth. Theellipsoidal subreflector has a major axis that is coincident withboresight axis 20, and on which lie a near focus 63 and a far focus 65that are positioned between the concave sides of the subreflector andmain dish 15. The subreflector S5 is positioned with its near focus 63coincident with the main dish focal point 27, and the feed 51 ispositioned at the subreflector far focus 65. Microwave source 21supplies energy to feed 51 via conduit 23 and switch 57 in response to awide beamwidth command issued to the switch 57. The feed 51 illuminatesthe subreflector with a cone of radiation having a small included angle,6 which optimally just subtends the subreflector. This cone of radiationis bounded by rays 67a and 690. Due to the concavity of thesubreflector, the radiation reflected therefrom, defined by rays 67b and69b, goes through a focus at the subreflector near focal point 63.

.was found experimentally that a 4-wavelength displacement of the feedtoward the ellipsoidal subreflector was adequate. This cone ofradiationstrikes the dish 15 at a reduced radius. A reduced radiuscollimated beam bounded by rays 69c and 670 is produced because thesubreflector is truncated or of too smalla size for the feed toilluminate the outer annular region 18 of the main dish. To illuminatethe entire main reflector from feed 51 via subreflector 5 5, thesubreflector would have to be larger by an annular region 71 shown inFIG. 1. Thus, in this embodiment, beamwidth switching is accomplished byelectrically switching microwave excitation from source 21 between feed53 for wide beamwidth and feed 51 for narrow beamwidth.

Practical dimensions for the inventive embodiment have been determinedin conjunction with a five foot focal length, 12.5 foot diameter mainparabolic dish 15 for 15 Gigahertz (0.0656 foot wavelength) operation.The narrow beamwidth of such a dish without subrefiector truncation isapproximately 030.

For this embodiment, the near focus 63 of ellipsoid subreflector 55 is1.25 feet in front of the subreflector, and its further focus 65 is 5.00feet in front of the subreflector. If the subreflector were nottruncated, it would be 2.8 feet in diameter. Truncation of the feed toapproximately two feet in diameter yields a 50% increase in beamwidth.

FIG. 3 is a design curve for the embodiment with the beamwidth indegrees as ordinate and the diameter of the subreflector 55 as abscissa.As can be seen, a smooth curve is obtained which can yield amultiplication of the beamwidth by a factor up to three as thesubreflector is truncated. This Gregorian embodiment 49 is particularlyattractive because of the instantaneous and reliable nature of theelectronic switching of microwave switch 57.

As mentioned earlier, geometric optic principles are approximate innature and diffraction theory must be resorted to in order to fullydescribe the invention. The numerical embodiments of the invention wereverified using a computer simulation which accounted for the diffractioneffects between the subreflector and the main dish and for thediffraction effects at the main dish. The subreflectors were divided upinto plural square regions, each having sides of 0.3 wavelengths, andthe main dish was also divided into plural square regions having sidesof 2 wavelengths. The square regions were considered to be coherenceareas. The radiation from a feed with assumed radiation patterns wascollected in each square region on the subreflector and a resultantsource obtained for each square. The radiation from each of thesesources was collected in each square of the main reflector and a secondset of sources on the main reflector was thereby obtained. The patternof far field radiation from these second sources was then calculated toobtain the far field pattern of the antenna with various sizedsubreflectors. These patterns verified that the design curve of FIG. 3is indeed a smooth curve.

FIG. 4 shows a nested multifrequency feed 111 adapted to be used withthe invention. Four large 5 band horns 1 13 abut to define a square fourfeed array centered about axis 20. Nested at the interior corners of thehorns 113 are four smaller X-band horns 115 which abut to form a smallersquare array centered about axis 20. The antenna of the invention isadaptable to multifrequency operation by using such a nested feed. Also,the square four feed array is adaptable to either amplitude or phasesimultaneous lobing or monopulse techniques for determining antennapointing errors. For simultaneous lobing, it is important that the phaseand amplitude of signals received at each of the four horns be equalwhen the transmitter of the signal is located on axis 20. It is furtherimportant that these parameters smoothly vary as the transmitting sourcedeparts from the antenna axis by an angle. Computer simulation hasverified that these parameters vary smoothly for source angles rangingfrom zero to well in excess of half the beamwidth.

Having described an embodiment of my invention it is clear that numerousmodifications of this embodiment are possible within the inventionsspirit and scope. Of particular import is the fact that antennas arereciprocal devices useful for transmitting, receiving, or both. It isintended that the invention not be limited except by the followingclaims which are generic to transmitting and/or receiving antennas.

What is claimed is:

1. A switchable beamwidth antenna having a wide beamwidth state and anarrow beamwidth state at a single predetermined frequency, saidbeamwidth state being responsive to a beamwidth selection command sourcecomprising:

a main reflecting dish having a boresight axis and a direction of viewalong said axis, wherein said dish may be divided with an imaginarycontour coaxial with the boresight axis into a central portion and anouter portion bordering the central portion; and

single frequency means responsive to the beamwidth selection commandsource, exciting at said single predetermined frequency substantiallyonly the central portion of the main dish in the wide beamwidth state,and exciting both the central and outer portions of the main dish in thenarrow beamwidth state, said single frequency means including in thewide beamwidth state a truncated subreflector positioned to excite themain dish and further including a first feed positioned to view saidtruncated subreflector the truncated subreflector having a concavereflecting surface facing the main dish, being positioned to be excitedby the first feed, and being of a size that only the central portion ofthe main dish may be viewed by the first feed via the subreflector, saidsingle frequency means including in the narrow beamwidth state a secondfeed located between the main reflector and the subreflector andpositioned to directly view substantially all of the main dish.

A radio frequency conduit coupled to a microwave source or receiver anda radio frequency switch means responsive to the beamwidth selectioncommand for coupling the radio frequency conduit to the first feed inthe wide beamwidth state and to the second feed in the narrow beamwidthstate.

2. An electronically selectable beamwidth antenna comprising:

a concave paraboloid main reflector having an axis and a focal point,and where said main reflector has a central circular region and an outerannular region bordering the central region;

a concave ellipsoid subreflector coaxial with and facing the concavemain reflector, the subreflector having a first focus substantiallycoincident with the main reflector focal point for directly exciting themain reflector and a second focus on the axis for exciting the mainreflector via the subreflector, the subreflector being of such size thatonly the central portion of the main reflector can be viewed via thesubreflector;

first feed means, located at the first focus of the subreflector fordirectly exciting the main reflector at a predetermined frequency;

second feed means, located at the second focus of the subreflector forexciting, via the subreflector, only the central portion of the mainreflector at said predetermined frequency; and

switch means for operatively selecting the first or second feed means.

1. A switchable beamwidth antenna having a wide beamwidth state and anarrow beamwidth state at a single predetermined frequency, saidbeamwidth state being responsive to a beamwidth selection command sourcecomprising: a main reflecting dish having a boresight axis and adirection of view along said axis, wherein said dish may be divided withan imaginary contour coaxial with the boresight axis into a centralportion and an outer portion bordering the central portion; and singlefrequency means responsive to the beamwidth selection command source,exciting at said single predetermined frequency substantially only thecentral portion of the main dish in the wide beamwidth state, andexciting both the central and outer portions of the main dish in thenarrow beamwidth state, said single frequency means including in thewide beamwidth state a truncated subreflector positioned to excite themain dish and further including a first feed positioned to view saidtruncated subreflector the truncated subreflector having a concavereflecting surface facing the main dish, being positioned to be excitedby the first feed, and being of a size that only the central portion ofthe main dish may be viewed by the first feed via the subreflector, saidsingle frequency means including in the narrow beamwidth state a secondfeed located between the main reflector and the subreflector andpositioned to directly view substantially all of the main dish. A radiofrequency conduit coupled to a microwave source or receiver and a radiofrequency switch means responsive to the beamwidth selection command forcoupling the radio frequency conduit to the first feed in the widebeamwidth state and to the second feed in the narrow beamwidth state. 2.An electronically selectable beamwidth antenna comprising: a concaveparaboloid main reflector having an axis and a focal point, and wheresaid main reflector has a central circular region and an outer annularregion bordering the central region; a concave ellipsoid subreflectorcoaxial with and facing the concave main reflector, the subreflectorhaving a first foCus substantially coincident with the main reflectorfocal point for directly exciting the main reflector and a second focuson the axis for exciting the main reflector via the subreflector, thesubreflector being of such size that only the central portion of themain reflector can be viewed via the subreflector; first feed means,located at the first focus of the subreflector for directly exciting themain reflector at a predetermined frequency; second feed means, locatedat the second focus of the subreflector for exciting, via thesubreflector, only the central portion of the main reflector at saidpredetermined frequency; and switch means for operatively selecting thefirst or second feed means.